Duplex hydraulic systems are used for example in aircraft actuator systems for redundancy and safety so that if one system fails, the other remains operational, allowing continued control of the relevant system. For example the main rotor actuator in a helicopter typically uses a duplex control system. Failure modes may include seal failures (leaks), pipe bursts, component structural failure or pump failure. Triplex and even quadruplex systems are also used in some applications.
Hydraulic spool valves in Flight Control actuators are usually driven by a mechanical lever connected to the pilots input lever which is in turn connected to the pilot's controls by a mechanical linkage. Where duplex hydraulic systems are employed for redundancy and safety reasons, two valves are used, one for each system, driven by a single layshaft and lever assembly. Synchronization of these two valves is critical to avoid potentially damaging ‘force fight’ between the two hydraulic systems. Force fight is created by the two valves being out of synchronization and this can lead to pressure intensification within the actuator. This intensification can cause premature seal failures and may also cause fatigue damage within the actuator. Any backlash or clearances due to manufacturing tolerances between the drive levers and the spools is exploited and can lead to unequal base pressures which in turn can cause unsatisfactory performance. Force fight can occur in actuators with tandem cylinder configurations as shown in FIG. 3. The four chamber pressures within the cylinder need to be precisely controlled so as to minimise the internal forces generated within the tandem piston and cylinder arrangement. These chamber pressures (referred to as base pressures) are controlled by very small spool displacements (around 0.001 inch) and therefore any small amount of deviation in the valve synchronization between the two hydraulic systems can result in damaging force fights.
It is therefore necessary to manufacture to extremely tight tolerances to ensure zero or minimum clearances.